I waited a few days hoping someone may be able to explain as I have noted this peculiarity. I am not sure there is a fixed rule or explanation for «cheveux longs». «longues dents pointues follows the 'rules'»

Long/ue/s usually goes before nouns. However, in usage and in many references  «cheveux longs», «bras long/s», and even «dents longs» are among a number of regular exceptions. My wife is French, and she spontaneously said all 3 of these, but reverted to «longues dents pointues». 

For the exceptions that I have seen, the only factor I can identify is that they refer to body parts/clothing (vétements longs is another I have seen). However, when there are multiple adjectives in these settings, «long» reverts to its usual position in front of these nouns in most of the circumstances I have observed. 

As my wife spontaneously changes the position of 'long', very much in keeping with the listings in 'word reference', it seems that it is quite common and just 'known'. I was intrigued to find almost every reference to «cheveux longs» expressed as such, and not  a word on this in the many explanations on adjectival positioning I checked.

Hopefully someone can add further insights.

The only explanation I've seen, is that before the noun, "long" tends to emphasize the size, but after the noun it describes the shape. So "jupe longue" or "cheveux longs" are, I suppose, more about the style than the length. Whether that can be applied to "dents longues", I'm not sure. Sometimes the order of adjectives is just based on what sounds best.